What is "managed code"?
When working with .NET, you will often encounter the term "managed code". This document will explain what this term means and additional information around it.
To put it very simply, managed code is just that: code whose execution is managed by a runtime. In this case, the runtime in question is called the Common Language Runtime or CLR, regardless of the implementation (for example, Mono, .NET Framework, or .NET Core/.NET 5+). CLR is in charge of taking the managed code, compiling it into machine code and then executing it. On top of that, runtime provides several important services such as automatic memory management, security boundaries, type safety etc.
Contrast this to the way you would run a C/C++ program, also called "unmanaged code". In the unmanaged world, the programmer is in charge of pretty much everything. The actual program is, essentially, a binary that the operating system (OS) loads into memory and starts. Everything else, from memory management to security considerations are a burden of the programmer.
Managed code is written in one of the high-level languages that can be run on top of .NET, such as C#, Visual Basic, F# and others. When you compile code written in those languages with their respective compiler, you don't get machine code. You get Intermediate Language code which the runtime then compiles and executes. C++ is the one exception to this rule, as it can also produce native, unmanaged binaries that run on Windows.
Intermediate Language & execution
What is "Intermediate Language" (or IL for short)? It is a product of compilation of code written in high-level .NET languages. Once you compile your code written in one of these languages, you will get a binary that is made out of IL. It is important to note that the IL is independent from any specific language that runs on top of the runtime; there is even a separate specification for it that you can read if you're so inclined.
Once you produce IL from your high-level code, you will most likely want to run it. This is where the CLR takes over and starts the process of Just-In-Time compiling, or JIT-ing your code from IL to machine code that can actually be run on a CPU. In this way, the CLR knows exactly what your code is doing and can effectively manage it.
Intermediate Language is sometimes also called Common Intermediate Language (CIL) or Microsoft Intermediate Language (MSIL).
Unmanaged code interoperability
Of course, the CLR allows passing the boundaries between managed and unmanaged world, and there is a lot of code that does that, even in the Base Class Libraries. This is called interoperability or just interop for short. These provisions would allow you to, for example, wrap up an unmanaged library and call into it. However, it is important to note that once you do this, when the code passes the boundaries of the runtime, the actual management of the execution is again in the hand of unmanaged code, and thus falls under the same restrictions.
Similar to this, C# is one language that allows you to use unmanaged constructs such as pointers directly in code by utilizing what is known as unsafe context which designates a piece of code for which the execution is not managed by the CLR.